Ever greater demands with respect to machining accuracy are being placed on modern machine tools. Therefore, in controlling such machine tools using numerical controls, procedures must be put in place to avoid machining errors.
Reproducible deviations of the finished workpiece from the desired workpiece form often have their cause in the kinematics of the machine tool, which always exhibits certain errors. To eliminate such reproducible, static errors, European Published Patent Application No. 1 189 121 describes measuring the positioning error of a tool at a number of working points in a working space, and entering it in a multidimensional compensation table. The compensation values from the compensation table, interpolated as needed, are then applied to the setpoint positions of the tool predefined in workpiece coordinates in a machining program, the setpoint positions only then being transferred to the numerical control for positioning the tool. However, this method does not take into account that the static error of a machine tool may be a function of the position of certain machine axes, such as axes of rotation.
U.S. Pat. No. 5,357,450 describes that linear axes and their errors in machine tools are still comparatively easy to measure and adjust, while the errors due to axes of rotation or swivel axes cause a considerably higher expenditure for measuring and adjusting. U.S. Pat. No. 5,357,450 describes an automated method by which such errors may be measured and converted into instructions for maintenance operations.
A different approach may be taken according to European Patent No. 0 289 836, which describes a comparatively complex method for analyzing the errors in a robot mechanism and taking them into account by mathematical methods when positioning the robot.
In addition, it is conventional to describe the kinematics of a machine tool with the aid of a kinematic table. Based on such a kinematic table, a numerical control is able to convert the setpoint positions of a tool, predefined in workpiece coordinates, into setpoint positions of the machine axes. The machine tool is completely described with respect to its geometry and its machine axes in this kinematic table. Transformations of the coordinates are indicated in the kinematic table in a plurality of entries describing the kinematics of the machine tool, by specifying an axial direction and an associated transformation amount, respectively, per entry. The machine axes available are likewise specified at the corresponding places in the kinematic table. An example for such a kinematic table is described further below.
By converting the setpoint position of the tool (i.e., its tool center point, TCP, and its orientation), indicated in workpiece coordinates, into axial coordinates or axial positions of the machine tool, a parts program written in workpiece coordinates is able to be executed on different machine tools, provided they are able in principle to move to all desired tool positions.